The invention relates to the purification of air intended to be cyrogenically fractionated, of the type in which the air is dried, decarbonated and at least partially stripped of the secondary atmospheric contaminates, such as hydrocarbons, nitrogen oxides, etc., by passing it through one or more bodies of adsorbent.
Conventionally, the body of adsorbent is cyclically regenerated by heating and/or flushing before it is used again in the adsorption phase.
At the present time, two types of processes are more particularly used for this purpose, namely:
PSA processes in which most of the regeneration power is provided by a pressure effect; and
TSA processes in which most of this same regeneration power is provided by a temperature effect.
Less conventionally, hybrid solutions have also been proposed for this purpose, especially in document U.S. Pat. No. 5,614,000.
Usually, a TSA (Temperature Swing Adsorption) process for purifying air comprises the following steps:
a) purification of the air by adsorption of the impurities at superatmospheric pressure and at ambient temperatures;
b) depressurization of the adsorber down to atmospheric pressure;
c) regeneration of the adsorbent at atmospheric pressure, especially by the residual or waste gases, typically impure nitrogen coming from an air separation unit and heated to a temperature above ambient by means of one or more heat exchangers;
d) cooling the adsorbent down to ambient or subambient temperature, especially by continuing to introduce air into the waste gas coming from the air separation unit, but this air not being heated;
e) repressurization of the adsorber with purified air coming, for example, from another adsorber in production phase.
Less conventionally, the regeneration may be carried out at a pressure substantially different from atmospheric pressure, either greater or even less than the latter by using, in this case, suitable vacuum pumping means.
As regards a PSA (Pressure Swing Adsorption) process cycle for purifying air, this comprises substantially the same steps a), b) and e) but is distinguished from a TSA process by the absence of heating of the waste gas or gases during the regeneration step (step c), and therefore the absence of step d), and, in general, a shorter cycle time than in the TSA process.
In general, the air pretreatment devices comprise two adsorbers, operating alternately, that is to say one of the adsorbers is in production phase while the other is in regeneration phase.
Such TSA air purification processes are described for instance in document U.S. Pat. No. 3,738,084.
The process, whether it is a PSA or TSA process, may involve one or more adsorbent beds, that is to say a multi-bed process.
The adsorbents used are, without being limiting, zeolites, activated aluminas, silica gels, exchanged zeolites, doped aluminas, etc.
Furthermore, depending on the case, the adsorbers may have their axis vertical or horizontal, or else they may be of the radial type, etc.
However, in all cases, the objective of the purification is to stop the H2O and CO2 impurities, and the other possible contaminants likely to be present in the gas stream, down to contents compatible with the proper operation of the cryogenic unit, whatever the performance or safety level of the equipment.
This is because, in the absence of such an air purification treatment for removing its CO2 and water impurities, there will be condensation and/or solidification of these impurities while cooling the air to cryogenic temperatures, which as a result may cause blockage problems in the equipment, especially the heat exchangers, distillation columns etc.
Furthermore, it is also common practice to remove, at least partially, the hydrocarbons and nitrogen oxide impurities liable to be present in the air so as to avoid any risk of damaging the equipment, particularly the distillation column or columns located downstream of the cold box.
Typically, the maximum values of impurity contents permitted are, at peak, less than 1 vpm and, on average, substantially less than this value of 1 vpm.
Usually, the various operating conditions that may be encountered on a site are taken into account when designing the unit so as to ensure that the cryogenic distillation unit located downstream of the air purifiers, this unit generally being called the ASU (Air Separation Unit), operates properly under all circumstances.
The improvements made hitherto to this type of process relate:
either to the selection of the adsorbents: nature, multi-bed, particle size etc.;
or to the processes themselves: regeneration temperature, choice of number of adsorbers, etc.;
or to reducing the energy in a given process.
With regard to the improvements made to the adsorbents or to the processes themselves, mention may especially be made of the following documents: U.S. Pat. No. 5,531,808, U.S. Pat. No. 5,587,003, U.S. Pat. No. 4,233,038, U.S. Pat. No. 5,232,474, EP-A-744,205, EP-A-590,946, EP-A909,823, EP-A-909,824, EP-A-909,825, EP-A-862,937, EP-A-862,936, EP-A-766,991, EP-A-766,989 and EP-A-862,938.
Moreover, with regard to the improvements made to the reduction in energy consumed, mention may be made of the documents: U.S. Pat. No. 4,472,178 relating to the recovery of the adsorption heat in a regenerator; U.S. Pat. No. 4,627,856 relating to a process allowing the necessary energy to be reduced by separately regenerating the adsorbent for stopping the water and the adsorbent for stopping the CO2; and U.S. Pat. No. 5,766,311 relating to a regeneration process using multiple thermopulses.
The use of adsorbers in which the gas stream flows radially, allowing the head losses to be reduced, may also fall within this category, the reduction in energy then relating to the compression of the air.
All these methods therefore consist in using additional items of equipment: regenerator, internal heater, connecting pipework, etc.
Consequently, it will be immediately understood that these processes increase the complexity of the equipment.
It follows that the resulting increase in the costs of the plant must therefore be more than compensated for by energy saving so that these processes are useful from an industrial standpoint.
In contrast, the present invention aims to improve the known gas purification processes, particularly air purification processes, by appreciably reducing the amount of energy consumed.
In other words, the present invention aims to reduce the amount of energy consumed in the known gas purification processes no longer by adding expensive items of equipment but by optimizing the regeneration conditions at each cycle.
Put another way, the object of the present invention is to modify the operating conditions of a TSA-type gas purification process according to certain external parameters, such as the environmental conditions for example, so as to save energy by profiting from operating conditions which are more favourable than those considered during the design work, as conventionally done in the prior art, that is to say by using new calculation and regulation methods rather than new equipment.
Indeed, although a number of documents relating to processes for regulating gas purification units using TSA processes are known, none of them makes it possible to achieve performance levels as high as those obtained by the present invention.
By way of example, mention may be made of the documents:
U.S. Pat. No. 3,808,773 which describes a process comprising a step of regenerating the adsorbent, beginning as soon as the CO2 impurities break through, that is to say as soon as the bed of adsorbent is saturated by these CO2 impurities. The regeneration conditions undergo no modification or variation during this regeneration step;
U.S. Pat. No. 4,472,178 which specifies that the adsorption step terminates when the concentration of CO2 impurities in the purified air reaches 1 vpm. Here again, the regeneration conditions undergo no modification or adjustment during the regeneration step;
U.S. Pat. No. 5,531,808 which teaches a TSA-type unit equipped with conventional control and regulation means, that is to say with means for regulating the gas flow rates, in order to operate continuously in an efficient manner. According to that document, the adsorption step terminates when the CO2 front reaches a defined point in the bed of adsorbent. No method allowing an energy saving to be made is described therein;
U.S. Pat. No. 5,766,311 which relates to regeneration by multiple thermal pulses with selection of the waste gas flow rate and of the temperature variations, so as to control the desorption time. No regulation according to the operating conditions is described therein; and
U.S. Pat. No. 5,846,295 which teaches control of the TSA unit based on the input temperature being regulated by means of a programme for controlling the duration and suitable means for controlling valves.
Thus, it will be understood that, hitherto, the known regulating processes have relied on satisfactory operation of the purification unit (no water breakthrough), (no CO2 breakthrough, etc.), or on conventional regulation of the temperatures and/or flow rates.
As explained above, the object of the present invention is therefore to improve these known processes by an effective adaptation of the purification process not only in order to stop the impurities satisfactorily but also to save energy, as soon as the actual operating conditions so allow, by modifying the operating parameters of the purification cycle.
The invention therefore relates to a process for purifying a gas stream to be purified, containing at least one impurity chosen from carbon dioxide (CO2) and water vapour (H2O), comprising:
at least one step of adsorbing, on at least one bed of adsorbent contained in at least one adsorber, at least some of the impurities contained in the gas stream to be purified, the gas stream to be purified being at an adsorption temperature (Ta), and
at least one step of regenerating at least some of the bed of adsorbent contained in the at least one adsorber with at least one regeneration gas introduced into the at least one adsorber, the regeneration temperature (Tr) of the regeneration gas being greater than the temperature (Ta) of the gas stream to be purified,
characterized in that at least one energy parameter, chosen from the flow rate of the regeneration gas entering and/or leaving the at least one adsorber, the duration of the regeneration step and the regeneration temperature (Tr) of the regeneration gas entering the at least one adsorber, is controlled, modified and/or regulated depending on at least one operating condition chosen from the pressure of the gas stream to be purified entering and/or leaving the at least one adsorber, the flow rate of the gas stream to be purified entering and/or leaving the at least one adsorber, the temperature (Ta) of the gas stream to be purified entering the at least one adsorber and/or the content of at least one of the impurities contained in the gas stream to be purified entering the at least one adsorber and depending on the thermal profile of the heat front output by at least one adsorber at the end of regeneration.
Depending on the case, the process of the invention may include one or more of the following characteristics:
it includes at least one step of direct or indirect determination of at least one of the operating conditions;
at least one of the operating conditions is determined directly by at least one measurement of:
the pressure of the gas stream to be purified entering and/or leaving at least one adsorber, preferably by means of a pressure gauge;
the flow rate of the gas stream to be purified entering and/or leaving at least one adsorber, preferably by means of a flowmeter;
the temperature (Ta) of the gas stream to be purified entering at least one adsorber, preferably by means of a thermometer or a temperature sensor; and/or
the content of at least one impurity in the gas stream to be purified entering at least one adsorber, preferably by means of a gas analyser;
at least one of the operating conditions is determined indirectly by at least one measurement taken from amongst:
one or more atmospheric conditions, preferably one or more atmospheric conditions chosen from the group formed by the temperature, the pressure and/or the relative humidity;
one or more conditions at the intake of at least one gas compressor which compresses the gas to be purified, preferably one or more conditions at the intake of the compressor which are chosen from the group formed by the temperature, the pressure and/or the relative humidity;
one or more pressure and/or temperature conditions at least within at least one fluid/liquid separator, the liquid to be separated being essentially atmospheric water and/or refrigeration water coming from direct contact heat exchange;
one or more temperature conditions in at least one bed of adsorbent; and
one or more operating conditions of a gas compressor, particularly the energy consumed and/or the speed of rotation of the compressor;
at least one energy parameter is corrected according to the thermal profile as the heat front leaves, preferably by measuring the maximum temperature as the said heat front leaves and by comparing this measured maximum temperature with a predetermined target temperature;
the adsorption pressure is between 2 bar and 30 bar; and/or
the regeneration pressure is between 1 bar and 30 bar; and/or
the adsorption temperature (Ta) is between 0xc2x0 C. and +80xc2x0 C.; and/or
the regeneration temperature (Tr) is between 20xc2x0 C. and 200xc2x0 C.;
the ratio of the regeneration rate to the adsorption rate is between 5% and 80%;
it includes at least one step of cryogenically distilling the purified gas stream leaving at least one adsorber;
the gas stream to be purified is air;
the regeneration gas is essentially nitrogen or a gas stream containing oxygen;
at least one bed of adsorbent contains an X zeolite having an Si/Al ratio ranging from approximately 1 to approximately 1.5 and/or at least one bed of activated alumina particles;
at least one bed of activated alumina articles is located upstream of at least one bed of X zeolite, preferably a bed of LSX zeolite having an Si/Al ratio of approximately 1;
it is chosen from among TSA processes;
the alumina is an alumina impregnated with a solution of alkali or alkaline-earth metal salts, preferably the alumina contains less than 10% by weight of one or more alkali or alkaline-earth metals and, in particular, the metal or metals are chosen from the group formed by sodium (Na+), potassium (K+) and calcium (Ca2+)
the alumina contains at least 1% by weight of one or more alkali or alkaline-earth metals, preferably from 2 to 9.8% by weight of one or more alkali or alkaline-earth metals, and more preferably at least 3.5% by weight of one of more alkali or alkaline-earth metals;
the X zeolite, having an Si/Al ratio  less than 1.15 and preferably about 1, is exchanged with or contains less than 35% potassium cations (K+), from, 1 to 99% sodium cations (Na+), and less than 99% calcium cations (Ca2+), preferably from 0.01% to 12% potassium cations (K+), from 1 to 99% calcium cations (Ca2+) and from 1 to 99% sodium cations (N+);
the X zeolite, having an Si/Al ratio  less than 1.15 and preferably about 1, is exchanged with or contains less than 10% potassium cations (K+), from 1 to 50% sodium cations (Na+) and from 50 to 99% calcium cations (Ca2+), preferably at least 66% calcium cations (Ca+) and even more preferably from 80 to 96% calcium cations;
the X zeolite, having an Si/Al ratio  less than 1.15 and preferably about 1, is exchanged with or contains from 0 to 7% potassium cations (K+), from 4 to 11% sodium cations (Na+) and from 82 to 92% calcium cations (Ca2+);
the X zeolite, having an Si/Al ratio  less than 1.15 and preferably about 1, is exchanged with or contains, in addition, from 0 to 98% lithium cations (Li+), preferably from 60 to 96% lithium cations.
In addition, the invention also relates to a gas purification system, comprising:
at least one adsorber containing at least one bed of adsorbent;
at least one source of a gas stream to be purified, containing at least one impurity chosen from carbon dioxide (CO2) and water vapour (H2O);
at least one source of regeneration gas;
means for feeding at least the adsorber, in order to introduce or feed, alternately, the at least one adsorber;
with the gas stream to be purified, in order to adsorb at least some of the impurities contained in the gas stream to be purified on the adsorbent, and
with the regeneration gas, in order to desorb at least some of the impurities adsorbed on the adsorbent;
control means for controlling, modifying and/or regulating at least one energy parameter chosen from the flow rate of the regeneration gas entering and/or leaving the at least one adsorber, the duration of the regeneration step and the regeneration temperature (Tr) of the regeneration gas entering the at least one adsorber, depending on at least one operating condition chosen from the pressure of the gas stream to be purified entering and/or leaving the at least one adsorber, the flow rate of the gas stream to be purified entering and/or leaving the at least one adsorber, the temperature (Ta) of the gas stream to be purified entering the at least one adsorber and the content of at least one of the impurities contained in the gas stream to be purified entering the at least one adsorber, and depending on the thermal profile of the heat front output by at least one adsorber at the end of regeneration.
Preferably, the system includes at least two adsorbers operating alternately, one being in regeneration phase while the other is in adsorption phase, that is to say the phase during which gas, in particular purified air, is produced.
By virtue of the present invention, major energy savings may be achieved by benefiting, depending on the cases encountered:
from a change in the operating conditions of other items of equipment generally located upstream of the purification, such as compressors, atmospheric exchangers, water-cooled air chiller, refrigerating unit; and/or
from a reduction in the losses inherent in the process, such as for example the influx of heat into equipment operating at temperatures below ambient; and/or
from changing the running conditions of the downstream unit(s) such as operating at a reduced rate for example.
This is because, in general, the removal of water and CO2 is facilitated when the atmospheric temperature decreases.
The colder temperature of the compressed air leaving the final refrigeration stage of the compressor or of the refrigerating unit (which itself is favoured by a cold source at a lower temperature) limits the amount of water contained and therefore to be stopped by the adsorbent.
The CO2 adsorption temperature decreases not only because the input temperature is lower, but also because the amount of heating due to the adsorption of waterxe2x80x94in a smaller amount, is itself less.
The variations in the operation of a purification operation may be greater than merely the summer/winter and/or day/night environmental changes in the case of standard apparatuses intended to be used both in cold countries and in hotter countries, such as tropical countries.
Moreover, the air or fluid flow rate/pressure conditions ensuring regeneration may depend on the running of the unit and therefore the situation may therefore be one in which the purification is carried out under actual operating conditions which differ from the design conditions.
The present invention will now be more clearly understood with the aid of examples and with reference to the appended figures, given by way of illustration but implying no limitation.
This example relates to a standard-type air purification and separation apparatus producing pure nitrogen in low volume.